Abstract: Over the past few decades, there has been a significant increase in the incidence of Clostridium difficile infection (CDI) in patients suffering from inflammatory bowel disease (IBD), who then suffer from more severe disease outcomes. This is becoming a serious issue faced by clinicians in the management of the comorbidity. The current standard treatment for CDI is antibiotics, such as vancomycin, which disrupts gut microbiota and induces high rates of recurrence, up to 35%. Since both IBD and CDI are associated with dysbiosis of gut microbiota, a treatment approach that is not associated with the induction of microbiota dysbiosis is urgently needed to curb the rapidly increasing incidence of CDI comorbidity with IBD.!We propose to develop a non- traditional therapeutic against comorbidity of CDI in patients with IBD that does not promote bacterial resistance, a major concern to public health worldwide. Our goal is to develop a novel combinational immunotherapy by utilizing probiotic yeast Saccharomyces boulardii to deliver therapeutic antibodies against major C. difficile toxins and inflammatory mediator tumor necrosis factor alpha (TNF-?). We have already generated a lead S. boulardii strain (FZ002) stably secreting a tetra-specific toxin-neutralizing antibody called ABAB. This antibody simultaneously targets the two major virulence factors toxin A (TcdA) and toxin B (TcdB) at the site of C. difficile infection and exhibits significant protection against CDI. In this project, we will construct another S. boulardii strain to stably secrete an anti-TNF-? neutralizing antibody (Sb-aTNF). We will then evaluate therapeutic efficacy of oral FZ002 combined with Sb-aTNF for treating CDI comorbidity with IBD in mouse disease models. With this proof-of-concept study, we expect to demonstrate the feasibility of a novel immunotherapeutic modality based on probiotic yeast for oral delivery of therapeutic antibodies to intestines where the comorbidity of CDI and IBD occurs. This will justify a larger development effort in a Phase II study to generate a final lead of combinational yeast immunotherapy to treat the comorbidity of CDI and IBD.